Bone marrow adiposity and the hematopoietic niche: A historical perspective of reciprocity, heterogeneity, and lineage commitment

Role of joint adipose tissues in osteoarthritis

Osteoarthritis (OA) is the most common musculoskeletal disease, without any curative treatment. Obesity being the main modifiable risk factor for OA, much attention focused on the role of adipose tissues (AT). In addition to the involvement of visceral and subcutaneous AT via systemic ways, many arguments also highlight the involvement of local AT, present in joint tissues. Local AT include intra-articular AT (IAAT), which border the synovium, and bone marrow AT (BMAT) localized within marrow cavities in the bones. This review describes the known features and involvement of IAAT and BMAT in joint homeostasis and OA. Recent findings evidence that alteration in magnetic resonance imaging signal intensity of infrapatellar fat pad can be predictive of the development and progression of knee OA. IAAT and synovium are partners of the same functional unit; IAAT playing an early and pivotal role in synovial inflammation and fibrosis and OA pain. BMAT, whose functions have only recently begun to be studied, is in close functional interaction with its microenvironment. The volume and molecular profile of BMAT change according to the pathophysiological context, enabling fine regulation of haematopoiesis and bone metabolism. Although its role in OA has not yet been studied, the localization of BMAT, its functions and the importance of the bone remodelling processes that occur in OA argue in favour of a role for BMAT in OA.

Exploring contrast-enhancing staining agents for studying adipose tissue through contrast-enhanced computed tomography

Contrast-enhanced computed tomography offers a nondestructive approach to studying adipose tissue in 3D. Several contrast-enhancing staining agents (CESAs) have been explored, whereof osmium tetroxide (OsO4) is the most popular nowadays. However, due to the toxicity and volatility of the conventional OsO4, alternative CESAs with similar staining properties were desired. Hf-WD 1:2 POM and Hexabrix have proven effective for structural analysis of adipocytes using contrast-enhanced computed tomography but fail to provide chemical information. This study introduces isotonic Lugol's iodine (IL) as an alternative CESA for adipose tissue analysis, comparing its staining potential with Hf-WD 1:2 POM and Hexabrix in murine caudal vertebrae and bovine muscle tissue strips. Single and sequential staining protocols were compared to assess the maximization of information extraction from each sample. The study investigated interactions, distribution, and reactivity of iodine species towards biomolecules using simplified model systems and assesses the potential of the CESA to provide chemical information. (Bio)chemical analyses on whole tissues revealed that differences in adipocyte gray values post-IL staining were associated with chemical distinctions between bovine muscle tissue and murine caudal vertebrae. More specific, a difference in the degree of unsaturation of fatty acids was identified as a likely contributor, though not the sole determinant of gray value differences. This research sheds light on the potential of IL as a CESA, offering both structural and chemical insights into adipose tissue composition.

Bone Marrow Adipose Tissue

Bone marrow (BM) acts as a dynamic organ within the bone cavity, responsible for hematopoiesis, skeletal remodeling, and immune system control. Bone marrow adipose tissue (BMAT) was long simply considered a filler of space, but now it is known that it instead constitutes an essential element of the BM microenvironment that participates in homeostasis, influences bone health and bone remodeling, alters hematopoietic stem cell functions, contributes to the commitment of mesenchymal stem cells, provides effects to immune homeostasis and defense against infections, and participates in energy metabolism and inflammation. BMAT has emerged as a significant contributor to the development and progression of various diseases, shedding light on its complex relationship with health. Notably, BMAT has been implicated in metabolic disorders, hematological malignancies, and skeletal conditions. BMAT has been shown to support the proliferation of tumor cells in acute myeloid leukemia and niche adipocytes have been found to protect cancer cells against chemotherapy, contributing to treatment resistance. Moreover, BMAT's impact on bone density and remodeling can lead to conditions like osteoporosis, where high levels of BMAT are inversely correlated with bone mineral density, increasing the risk of fractures. BMAT has also been associated with diabetes, obesity, and anorexia nervosa, with varying effects on individuals depending on their weight and health status. Understanding the interaction between adipocytes and different diseases may lead to new therapeutic strategies.

Bone Marrow Adipose Tissue Is Not Required for Reconstitution of the Immune System Following Irradiation in Male Mice

Bone marrow adipose tissue (BMAT) is hypothesized to serve as an expandable/contractible fat depot which functions, in part, to minimize energy requirements for sustaining optimal hematopoiesis. We investigated whether BMAT is required for immune reconstitution following injury. Male wild type (WBB6F1, WT) and BMAT-deficient WBB6F1/J-KitW/KitW-v/J (KitW/W-v) mice were lethally irradiated. Irradiation was followed by adoptive transfer of 1000 purified WT hematopoietic stem cells (HSCs). The extent of immune reconstitution in blood, bone marrow, and lymph nodes in the irradiated mice was determined using HSCs from green fluorescent protein (GFP)-expressing mice. We also evaluated skeletal response to treatment. Detection of GFP-positive B and T cells in peripheral blood at 4 and 9 weeks following adoptive transfer and in bone marrow and lymph nodes following necropsy revealed excellent immune reconstitution in both WT and BMAT-deficient mice. Adipocytes were numerous in the distal femur of WT mice but absent or rare in KitW/W-v mice. Bone parameters, including length, mass, density, bone volume, microarchitecture, and turnover balance, exhibited few differences between WT and BMAT-deficient mice. The minimal differences suggest that BMAT is not required for reconstitution of the immune system following lethal radiation and is not a major contributor to the skeletal phenotypes of kit signaling-deficient mice.

MarrowCellDLD: a microfluidic method for label-free retrieval of fragile bone marrow-derived cells

Functional bone marrow studies have focused primarily on hematopoietic progenitors, leaving limited knowledge about other fragile populations, such as bone marrow adipocytes (BMAds) and megakaryocytes. The isolation of these cells is challenging due to rupture susceptibility and large size. We introduce here a label-free cytometry microsystem, MarrowCellDLD, based on deterministic lateral displacement. MarrowCellDLD enables the isolation of large, fragile BM-derived cells based on intrinsic size properties while preserving their viability and functionality. Bone marrow adipocytes, obtained from mouse and human stromal line differentiation, as well as megakaryocytes, from primary human CD34+ hematopoietic stem and progenitor cells, were used for validation. Precise micrometer-range separation cutoffs were adapted for each cell type. Cells were sorted directly in culture media, without pre-labeling steps, and with real-time imaging for quality control. At least 106 cells were retrieved intact per sorting round. Our method outperformed two FACS instruments in purity and yield, particularly for large cell size fractions. MarrowCellDLD represents a non-destructive sorting tool for large, fragile BM-derived cells, facilitating the separation of pure populations of BMAds and megakaryocytes to further investigate their physiological and pathological roles.

Mesenchymal stromal cells, metabolism, and mitochondrial transfer in bone marrow normal and malignant hematopoiesis

Hematopoietic stem cell (HSC) transplantation-based treatments are in different phases of clinical development, ranging from current therapies to a promise in the repair and regeneration of diseased tissues and organs. Mesenchymal stromal/stem cells (MSCs), which are fibroblast-like heterogeneous progenitors with multilineage differentiation (osteogenic, chondrogenic, and adipogenic) and self-renewal potential, and exist in the bone marrow (BM), adipose, and synovium, among other tissues, represent one of the most widely used sources of stem cells in regenerative medicine. MSCs derived from bone marrow (BM-MSCs) exhibit a variety of traits, including the potential to drive HSC fate and anti-inflammatory and immunosuppressive capabilities via paracrine activities and interactions with the innate and adaptive immune systems. The role of BM-MSC-derived adipocytes is more controversial and may act as positive or negative regulators of benign or malignant hematopoiesis based on their anatomical location and functional crosstalk with surrounding cells in the BM microenvironment. This review highlights the most recent clinical and pre-clinical findings on how BM-MSCs interact with the surrounding HSCs, progenitors, and immune cells, and address some recent insights on the mechanisms that mediate MSCs and adipocyte metabolic control through a metabolic crosstalk between BM microenvironment cells and intercellular mitochondrial transfer in normal and malignant hematopoiesis.

Good Cop, Bad Cop: Profiling the Immune Landscape in Multiple Myeloma

Multiple myeloma (MM) is a dyscrasia of plasma cells (PCs) characterized by abnormal immunoglobulin (Ig) production. The disease remains incurable due to a multitude of mutations and structural abnormalities in MM cells, coupled with a favorable microenvironment and immune suppression that eventually contribute to the development of drug resistance. The bone marrow microenvironment (BMME) is composed of a cellular component comprising stromal cells, endothelial cells, osteoclasts, osteoblasts, and immune cells, and a non-cellular component made of the extracellular matrix (ECM) and the liquid milieu, which contains cytokines, growth factors, and chemokines. The bone marrow stromal cells (BMSCs) are involved in the adhesion of MM cells, promote the growth, proliferation, invasion, and drug resistance of MM cells, and are also crucial in angiogenesis and the formation of lytic bone lesions. Classical immunophenotyping in combination with advanced immune profiling using single-cell sequencing technologies has enabled immune cell-specific gene expression analysis in MM to further elucidate the roles of specific immune cell fractions from peripheral blood and bone marrow (BM) in myelomagenesis and progression, immune evasion and exhaustion mechanisms, and development of drug resistance and relapse. The review describes the role of BMME components in MM development and ongoing clinical trials using immunotherapeutic approaches.

Loss of Notch signaling in skeletal stem cells enhances bone formation with aging

Skeletal stem and progenitor cells (SSPCs) perform bone maintenance and repair. With age, they produce fewer osteoblasts and more adipocytes leading to a loss of skeletal integrity. The molecular mechanisms that underlie this detrimental transformation are largely unknown. Single-cell RNA sequencing revealed that Notch signaling becomes elevated in SSPCs during aging. To examine the role of increased Notch activity, we deleted Nicastrin, an essential Notch pathway component, in SSPCs in vivo. Middle-aged conditional knockout mice displayed elevated SSPC osteo-lineage gene expression, increased trabecular bone mass, reduced bone marrow adiposity, and enhanced bone repair. Thus, Notch regulates SSPC cell fate decisions, and moderating Notch signaling ameliorates the skeletal aging phenotype, increasing bone mass even beyond that of young mice. Finally, we identified the transcription factor Ebf3 as a downstream mediator of Notch signaling in SSPCs that is dysregulated with aging, highlighting it as a promising therapeutic target to rejuvenate the aged skeleton.

A distinct class of hematopoietic stem cells develop from the human yellow bone marrow

Aging and metabolic diseases are accompanied by systemic inflammation, but the mechanisms that induce this state are not known. We developed a human bone-marrow organoid system to explore mechanisms underlying metabolic-disease associated systemic inflammation. We find that a distinct type of hematopoietic stem cell (HSC) develops in the adipose-rich, yellow bone marrow, which is known to gradually replace the hematopoietic red marrow as we age and during metabolic disease. Unlike HSCs derived from the red bone marrow, HSCs derived from the yellow bone marrow have higher proliferation rates, increase myeloid differentiation, skew towards pro-inflammatory M1 macrophage differentiation, and express a distinct transcriptomic profile associated with responsiveness to wounding. Yellow marrow-derived HSCs express higher levels of the leptin receptor, which we find to be further increased in patients with type 2 diabetes. Our work demonstrates that the human long bone yellow marrow is a niche for a distinct class of HSCs which could underlie hematopoietic dysfunction during aging and metabolic disease processes suggesting a shared inflammaging mechanism.

Distinct molecular profiles of skull bone marrow in health and neurological disorders

The bone marrow in the skull is important for shaping immune responses in the brain and meninges, but its molecular makeup among bones and relevance in human diseases remain unclear. Here, we show that the mouse skull has the most distinct transcriptomic profile compared with other bones in states of health and injury, characterized by a late-stage neutrophil phenotype. In humans, proteome analysis reveals that the skull marrow is the most distinct, with differentially expressed neutrophil-related pathways and a unique synaptic protein signature. 3D imaging demonstrates the structural and cellular details of human skull-meninges connections (SMCs) compared with veins. Last, using translocator protein positron emission tomography (TSPO-PET) imaging, we show that the skull bone marrow reflects inflammatory brain responses with a disease-specific spatial distribution in patients with various neurological disorders. The unique molecular profile and anatomical and functional connections of the skull show its potential as a site for diagnosing, monitoring, and treating brain diseases.

BHLHE40 Maintains the Stemness of PαS Cells In Vitro by Targeting Zbp1 through the Wnt/β-Catenin Signaling Pathway

Primary bone mesenchymal stem cells (BMSCs) gradually lose stemness during in vitro expansion, which significantly affects the cell therapeutic effects. Here, we chose murine PαS (SCA-1+PDGFRα+CD45-TER119-) cells as representative of BMSCs and aimed to explore the premium culture conditions for PαS cells. Freshly isolated (fresh) PαS cells were obtained from the limbs of C57/6N mice by fluorescence-activated cell sorting (FACS). We investigated the differences in the stemness of PαS cells by proliferation, differentiation, and stemness markers in vitro and by ectopic osteogenesis and chondrogenesis ability in vivo, as well as the changes in the stemness of PαS cells during expansion in vitro. Gain- and loss-of-function experiments were applied to investigate the critical role and underlying mechanism of the basic helix-loop-helix family member E40 (BHLHE40) in maintaining the stemness of PαS cells. The stemness of fresh PαS cells representative in vivo was superior to that of passage 0 (P0) PαS cells in vitro. The stemness of PαS cells in vitro decreased gradually from P0 to passage 4 (P4). Moreover, BHLHE40 plays a critical role in regulating the stemness of PαS cells during in vitro expansion. Mechanically, BHLHE40 regulates the stemness of PαS cells by targeting Zbp1 through the Wnt/β-catenin signaling pathway. This work confirms that BHLHE40 is a critical factor for regulating the stemness of PαS cells during expansion in vitro and may provide significant indications in the exploration of premium culture conditions for PαS cells.

Adipocyte- and Monocyte-Mediated Vicious Circle of Inflammation and Obesity (Review of Cellular and Molecular Mechanisms)

Monocytes play a key role in the development of metabolic syndrome, and especially obesity. Given the complex features of their development from progenitor cells, whose regulation is mediated by their interactions with bone marrow adipocytes, the importance of a detailed study of the heterogeneous composition of monocytes at the molecular and systemic levels becomes clear. Research argues for monocytes as indicators of changes in the body's metabolism and the possibility of developing therapeutic strategies to combat obesity and components of metabolic syndrome based on manipulations of the monocyte compound of the immune response. An in-depth study of the heterogeneity of bone-marrow-derived monocytes and adipocytes could provide answers to many questions about the pathogenesis of obesity and reveal their therapeutic potential.

Obesity and myeloma: Clinical and mechanistic contributions to disease progression

Obesity and obesogenic behaviors are positively associated with both monoclonal gammopathy of unknown significance (MGUS) and multiple myeloma (MM). As the only known modifiable risk factor, this association has emerged as a new potential target for MM prevention, but little is known about the mechanistic relationship of body weight with MM progression. Here we summarize epidemiological correlations between weight, body composition, and the various stages of myeloma disease progression and treatments, as well as the current understanding of the molecular contributions of obesity-induced changes in myeloma cell phenotype and signaling. Finally, we outline groundwork for the future characterization of the relationship between body weight patterns, the bone marrow microenvironment, and MM pathogenesis in animal models, which have the potential to impact our understanding of disease pathogenesis and inform MM prevention messages.

MarrowQuant 2.0: A Digital Pathology Workflow Assisting Bone Marrow Evaluation in Experimental and Clinical Hematology

Bone marrow (BM) cellularity assessment is a crucial step in the evaluation of BM trephine biopsies for hematologic and nonhematologic disorders. Clinical assessment is based on a semiquantitative visual estimation of the hematopoietic and adipocytic components by hematopathologists, which does not provide quantitative information on other stromal compartments. In this study, we developed and validated MarrowQuant 2.0, an efficient, user-friendly digital hematopathology workflow integrated within QuPath software, which serves as BM quantifier for 5 mutually exclusive compartments (bone, hematopoietic, adipocytic, and interstitial/microvasculature areas and other) and derives the cellularity of human BM trephine biopsies. Instance segmentation of individual adipocytes is realized through the adaptation of the machine-learning-based algorithm StarDist. We calculated BM compartments and adipocyte size distributions of hematoxylin and eosin images obtained from 250 bone specimens, from control subjects and patients with acute myeloid leukemia or myelodysplastic syndrome, at diagnosis and follow-up, and measured the agreement of cellularity estimates by MarrowQuant 2.0 against visual scores from 4 hematopathologists. The algorithm was capable of robust BM compartment segmentation with an average mask accuracy of 86%, maximal for bone (99%), hematopoietic (92%), and adipocyte (98%) areas. MarrowQuant 2.0 cellularity score and hematopathologist estimations were highly correlated (R² = 0.92-0.98, intraclass correlation coefficient [ICC] = 0.98; interobserver ICC = 0.96). BM compartment segmentation quantitatively confirmed the reciprocity of the hematopoietic and adipocytic compartments. MarrowQuant 2.0 performance was additionally tested for cellularity assessment of specimens prospectively collected from clinical routine diagnosis. After special consideration for the choice of the cellularity equation in specimens with expanded stroma, performance was similar in this setting (R² = 0.86, n = 42). Thus, we conclude that these validation experiments establish MarrowQuant 2.0 as a reliable tool for BM cellularity assessment. We expect this workflow will serve as a clinical research tool to explore novel biomarkers related to BM stromal components and may contribute to further validation of future digitalized diagnostic hematopathology workstreams.

Diversity in the bone marrow niche: Classic and novel strategies to uncover niche composition

Our view on the role and composition of the bone marrow (BM) has dramatically changed over time from a simple nutrient for the bone to a highly complex multicellular tissue that sustains haematopoiesis. Among these cells, multipotent haematopoietic stem cells (HSCs), which are predominantly quiescent, possess unique self-renewal capacity and multilineage differentiation potential and replenish all blood lineages to maintain lifelong haematopoiesis. Adult HSCs reside in specialised BM niches, which support their functions. Much effort has been put into deciphering HSC niches due to their potential clinical relevance. Multiple cell types have been implicated as HSC-niche components including sinusoidal endothelium, perivascular stromal cells, macrophages, megakaryocytes, osteoblasts and sympathetic nerves. In this review we provide a historical perspective on how technical advances, from genetic mouse models to imaging and high-throughput sequencing techniques, are unveiling the plethora of molecular cues and cellular components that shape the niche and regulate HSC functions.

Adipose tissue in bone regeneration - stem cell source and beyond

Adipose tissue (AT) is recognized as a complex organ involved in major home-ostatic body functions, such as food intake, energy balance, immunomodulation, development and growth, and functioning of the reproductive organs. The role of AT in tissue and organ homeostasis, repair and regeneration is increasingly recognized. Different AT compartments (white AT, brown AT and bone marrow AT) and their interrelation with bone metabolism will be presented. AT-derived stem cell populations - adipose-derived mesenchymal stem cells and pluripotent-like stem cells. Multilineage differentiating stress-enduring and dedifferentiated fat cells can be obtained in relatively high quantities compared to other sources. Their role in different strategies of bone and fracture healing tissue engineering and cell therapy will be described. The current use of AT- or AT-derived stem cell populations for fracture healing and bone regenerative strategies will be presented, as well as major challenges in furthering bone regenerative strategies to clinical settings.

Bone Marrow Niches of Hematopoietic Stem and Progenitor Cells

The mammalian hematopoietic system is remarkably efficient in meeting an organism’s vital needs, yet is highly sensitive and exquisitely regulated. Much of the organismal control over hematopoiesis comes from the regulation of hematopoietic stem cells (HSCs) by specific microenvironments called niches in bone marrow (BM), where HSCs reside. The experimental studies of the last two decades using the most sophisticated and advanced techniques have provided important data on the identity of the niche cells controlling HSCs functions and some mechanisms underlying niche-HSC interactions. In this review we discuss various aspects of organization and functioning of the HSC cell niche in bone marrow. In particular, we review the anatomy of BM niches, various cell types composing the niche, niches for more differentiated cells, metabolism of HSCs in relation to the niche, niche aging, leukemic transformation of the niche, and the current state of HSC niche modeling in vitro.

Leptin and environmental temperature as determinants of bone marrow adiposity in female mice

Bone marrow adipose tissue (BMAT) levels are higher in distal femur metaphysis of female mice housed at thermoneutral (32°C) than in mice housed at 22°C, as are abdominal white adipose tissue (WAT) mass, and serum leptin levels. We performed two experiments to explore the role of increased leptin in temperature-enhanced accrual of BMAT. First, we supplemented 6-week-old female C57BL/6J (B6) mice with leptin for 2 weeks at 10 µg/d using a subcutaneously implanted osmotic pump. Controls consisted of ad libitum (ad lib) fed mice and mice pair fed to match food intake of leptin-supplemented mice. The mice were maintained at 32°C for the duration of treatment. At necropsy, serum leptin in leptin-supplemented mice did not differ from ad lib mice, suggesting suppression of endogenous leptin production. In support, Ucp1 expression in BAT, percent body fat, and abdominal WAT mass were lower in leptin-supplemented mice. Leptin-supplemented mice also had lower BMAT and higher bone formation in distal femur metaphysis compared to the ad lib group, changes not replicated by pair-feeding. In the second experiment, BMAT response was evaluated in 6-week-old female B6 wild type (WT), leptin-deficient ob/ob and leptin-treated (0.3 μg/d) ob/ob mice housed at 32°C for the 2-week duration of the treatment. Compared to mice sacrificed at baseline (22°C), BMAT increased in ob/ob mice as well as WT mice, indicating a leptin independent response to increased temperature. However, infusion of ob/ob mice with leptin, at a dose rate having negligible effects on either energy metabolism or serum leptin levels, attenuated the increase in BMAT. In summary, increased housing temperature and increased leptin have independent but opposing effects on BMAT in mice.

Raman microspectroscopy reveals unsaturation heterogeneity at the lipid droplet level and validates an in vitro model of bone marrow adipocyte subtypes

Bone marrow adipocytes (BMAds) constitute the most abundant stromal component of adult human bone marrow. Two subtypes of BMAds have been described, the more labile regulated adipocytes (rBMAds) and the more stable constitutive adipocytes (cBMAds), which develop earlier in life and are more resilient to environmental and metabolic disruptions. In vivo, rBMAds are enriched in saturated fatty acids, contain smaller lipid droplets (LDs) and more readily provide hematopoietic support than their cBMAd counterparts. Mouse models have been used for BMAds research, but isolation of primary BMAds presents many challenges, and thus in vitro models remain the current standard to study nuances of adipocyte differentiation. No in vitro model has yet been described for the study of rBMAds/cBMAds. Here, we present an in vitro model of BM adipogenesis with differential rBMAd and cBMAd-like characteristics. We used OP9 BM stromal cells derived from a (C57BL/6xC3H)F2-op/op mouse, which have been extensively characterized as feeder layer for hematopoiesis research. We observed similar canonical adipogenesis transcriptional signatures for spontaneously-differentiated (sOP9) and induced (iOP9) cultures, while fatty acid composition and desaturase expression of Scd1 and Fads2 differed at the population level. To resolve differences at the single adipocyte level we tested Raman microspectroscopy and show it constitutes a high-resolution method for studying adipogenesis in vitro in a label-free manner, with resolution to individual LDs. We found sOP9 adipocytes have lower unsaturation ratios, smaller LDs and higher hematopoietic support than iOP9 adipocytes, thus functionally resembling rBMAds, while iOP9 more closely resembled cBMAds. Validation in human primary samples confirmed a higher unsaturation ratio for lipids extracted from stable cBMAd-rich sites (femoral head upon hip-replacement surgery) versus labile rBMAds (iliac crest after chemotherapy). As a result, the 16:1/16:0 fatty acid unsaturation ratio, which was already shown to discriminate BMAd subtypes in rabbit and rat marrow, was validated to discriminate cBMAds from rBMAd in both the OP9 model in vitro system and in human samples. We expect our model will be useful for cBMAd and rBMAd studies, particularly where isolation of primary BMAds is a limiting step.